ath5k: add missing iounmap to AHB probe removal

When our driver device is removed on the AHB bus, our IO memory is never unmapped.

Signed-off-by: Jonathan Bither <jonbither@gmail.com>
Reviewed-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>